1. Field of the Invention
This invention relates generally to the use of optical devices to sense the progress of processes, transmit signals related to such progress, detect such signals and extract from such signals information to control such processes. More particularly, this invention relates to an improved light collection system for use in optical emission spectroscopy devices designed to monitor, diagnose, and control processing and to determine endpoints in either photolithographic wet or dry etching processes or thin film processes for the fabrication of semiconductor devices.
2. Brief Description of the Prior Art
Photolithography makes it possible to transfer a desired circuit pattern to a surface of a semiconductor device. In a simplified photolithograpic process a silicon or gallium arsenide wafer or other substrate with a suitable substrate coating such as silicon dioxide, polysilicon, or aluminum or other metal is coated with a photoresist film and then subjected to an imaging and developing process which exposes regions of the substrate coating in a pattern defined by a mask having opaque and transparent portions positioned to form the desired pattern. The wafer is then etched by a subsequent process in the pattern formed by the developing process.
During the etching steps of the process it is important to monitor the progress of the etch and to detect the point at which the material or film which underlies the layer being etched is reached. Optical emission endpoint spectroscopy is currently used to monitor and detect process endpoint in plasma etching systems. This is possible because the plasma excites certain molecular species and causes them to emit light of wavelengths that are characteristic of each species. In an optical monitoring system specific wavelengths of the light emitted from the plasma are selected and fed to detectors, such as photo diodes, photo multipliers, and array detectors. It is known that the intensity of the signals is proportional to the level of light detected and by selecting wavelengths which correlate exclusively to either the reactants or the reaction products of the particular process, the process may be monitored. In particular, by selecting a wavelength which corresponds to the product of reactants from the process gas and the layer being etched, the point at which the next layer is reached may be easily detected by a decrease in measured emissions from the selected product. On the otherhand, by selecting a wavelength which corresponds to the reactants, the point at which the reactants are no longer being consumed is indicated by a change or stabilization in measured emissions from the reactants. When the film being etched has completely cleared from the underlying material or film there is a chemical change both in the gas phase and n the film. Product species from the film are no longer being generated, and some reactants increase because they are no longer being consumed by the reaction. These chemical changes show up as changes in optical emission intensities. Thus by continuously monitoring the intensity of an appropriate emission wavelength, either a reactant or product of the etch reaction, a change in emission intensity generally signals removal of the film being etched and contact of the etching agent with the underlying material or film. Although it is not uncommon to allow the reaction to proceed for an additional period of time, the point at which the underlying material or film is contacted is commonly known as endpoint.
In certain types of applications, including but not limited to plasma etching of semiconductor wafers, ion milling of surfaces, plasma diagnosis, and plasma deposition, where the light is emitted from an extended, diffuse anisotropic source and must be transmitted to a remote detector, it is necessary to collect as much light as possible. at the desired wavelengths to provide a sufficient signal to noise ratio for meaningful monitoring. The trend toward increased miniaturization of features causes the continued reduction of critical dimensions which in turn results in smaller area being reacted and thus less material producing emissions. This also makes it necessary to collect as much of the limited light as possible.
For purposes of this discussion an extended source is defined as a three dimensional source having a depth of the order of magnitude approximately equal to or greater than the distance between the source and the observation point. Diffuse light is defined as uncollimated light.
In a typical prior art system light would be transmitted from a chamber etch window to a detector by direct coupling of the detector to the window, by remotely coupling the detector to the window using a fiberoptic system without a focusing lens, or by remotely coupling the detector the window using a fiberoptic system with a spherical focusing lens. In many cases where the intensity of the light emission at the selected wavelengths is low, none of the above approaches collected sufficient light to optimally operate the endpoint controller with an extended diffuse anisotropic light source. To increase the light collection and delivery to the detector from such a source requires a system designed to adapt the inherent lower symmetry of the angular dispersion of light by this non-Lambertian source to collect light at the limiting etendue of the detector. Furthermore, it was determined that for certain processes multiple signals are desirable, and prior art approaches were so inadequate that endpoint detection was not deemed to be of practical value. Through experimentation it was determined that where the light source is asymmetric the light collection could be substantially improved by providing an anamorphic lens, which in effect matches the etendue of the source with the etendue of the detector.
The principal object of this invention is a device to collect light from extended diffuse anisotropic sources, such as plasma etchers, plasma deposition chambers, furnaces and similar sources and to deliver such light to a detector.
Another object of this invention is a device to condense a diffuse non-Lambertian light signal and provide a uniform distribution of intensity in the resulting pattern.
Yet another object of this invention is a device to provide the efficient transfer of light from a three dimensional, non-collimated source having an inherent fuzzy image to a detector.